1. Field of the Invention
The present invention relates to an electroluminescence display device employing an electroluminescence element and a thin film transistor.
2. Description of the Related Art
In recent years, electroluminescence (EL) display devices comprising EL elements have gained attention as potential replacement for CRT and LCD devices. Some research has been directed to the development of EL display devices using, for example, thin film transistors (referred to hereinafter as “TFT”) as switching elements to drive the EL elements.
FIG. 1 shows a plan view of a related organic EL display device.
As shown in the Figure, the organic EL display device comprises a display pixel region 200 having first and second TFTs for driving an organic EL element of the display pixel. The organic EL display device further comprises a peripheral drive circuit region 250 indicated by a single-dot broken line. The peripheral drive circuit region 250 includes vertical drive circuits 10 and horizontal drive circuit 20 for driving the TFT of the display pixel region.
FIG. 2 shows an equivalent circuit of a related single display pixel using an organic EL element. In the display pixel region 200, a single display pixel is surrounded by a gate signal line 151 and a drain signal line 152. A first TFT 130 is a switching element disposed near a junction of those lines. Source 131s of TFT 130 is connected to gate 142 of a second TFT 140 for driving the organic EL element 160. A storage capacitor 170 is provided between source 131s and gate 142 for retaining for a predetermined period a voltage applied to gate 142. Source 141s of the second TFT 140 is connected to the anode 161 of the organic EL element 160. Drain 141d of TFT 140 is connected to the drive power line 153 that supplies a drive current to the organic EL element 160.
FIG. 3 shows a cross-sectional structure including the second TFT 140 and the organic EL element 160 among components of a single display pixel. Gate electrodes 142 made of refractory metal such as chromium (Cr) or molybdenum (Mo) are formed on an insulator substrate 110 made of quartz glass, non-alkali glass, or a similar material. Sequentially formed over the gate electrodes 142 are a gate insulating film 112 and an active layer 141 using poly-silicon (referred to hereinafter as “p-Si”) film. The active layer 141 comprises intrinsic or substantially intrinsic channels 141c formed above the gate electrodes 142, and the source 141s and drain 141d formed on respective sides of these channels 141c by ion doping.
An interlayer insulating film 115 formed by a sequential deposit of a SiO2 film, a SiN film, and a SiO2 film covers the gate insulating film 112 and the active layer 141. A contact hole formed in the interlayer insulating film 115 in a region corresponding to the drain 141d is filled with metal such as aluminum (Al), forming the drive power line 153 connecting to a drive power supply 150. Further, a planarizing insulating film 117 made of an organic resin is formed over the entire substrate to planarize the surface. In a region corresponding to the source 141s, a contact hole is formed penetrating through both the planarizing insulating film 117 and the interlayer insulating film 115. A transparent electrode that contacts the source 141s through this contact hole is formed on the planarizing insulating film 117. The transparent electrode is made of ITO (indium tin oxide), and functions as the anode 161 of the organic EL element 160.
The organic EL element 160 is configured by sequentially forming, in order, the anode 161 made of ITO or similar material connected to the source 141s of the above-mentioned second TFT 140, an element emissive layer 166 composed using an organic compound, and a cathode 167 composed using an alloy of magnesium and indium. In such an organic EL element 160, a hole injected from the anode and an electron injected from the cathode recombines in an emissive layer within the element emissive layer 166. As a result, organic compound molecules in the emissive layer are excited, generating excitons. Through a process of these excitons undergoing radiation until deactivation, light is emitted from the emissive layer. This light radiates outward through the transparent anode 161 and the transparent insulator substrate 110.
As shown in FIG. 3, the anode 161 is discretely formed for each display pixel, and the element emissive layer 166 is formed slightly larger than the anode 161 so as to cover the entire anode 161. The cathode 167 can be formed as one common electrode over the entire substrate because the operation of the cathode can be electrically in common for all pixels. More specifically, as the cathode can be configured as a common electrode, according to the related art the cathode 167 can easily be provided by forming it in the region surrounded by a double-dot broken line in FIG. 1, this being the entire region of the substrate 110.
A TFT using poly-silicon as the active layer can be employed not only as a pixel TFT within the display pixel region 200, but also as a TFT for peripheral drive circuit to drive the display pixel region 200 on the substrate 110. In other words, circuit for driving the display pixel region 200 may be formed on the same substrate 110 as the pixel region. FIG. 4 illustrates a peripheral drive circuit disposed in a surrounding region of the display pixel region 200 as shown in FIG. 1, which is configured using the third TFT. The peripheral drive circuit is described below referring to FIGS. 1 and 4. Peripheral drive circuits configured using the third TFT comprise vertical drive circuits 10 and a horizontal drive circuit 20. A vertical drive circuit 10 includes a vertical shift register (V-SR) 11 and a buffer circuit 12, while a horizontal drive circuit 20 includes a horizontal shift register (H-SR) 21, a buffer 22, and a source line switch 23.
FIG. 4 is a plan view showing the TFT of the buffer constituting the horizontal drive circuit. FIG. 5 shows a cross-sectional view taken along line A—A of FIG. 4.
As shown in FIG. 4, the buffer comprises inverters 400 and 500.
The configuration of the respective TFT of the buffer is next described according to FIG. 5.
Sequentially formed on an insulator substrate 510 composed of a material such as silica glass or non-alkali glass are gate electrodes 511 made of refractory metal such as chromium (Cr) or molybdenum (Mo), a gate insulating film 512, and an active layer 513 composed of poly-silicon film.
The active layer 513 comprises channels 515,516 positioned above the gate electrodes 511. Further within the active layer, sources 518,521 and drains 519,520 are formed on respective sides of these channels 515,516 by performing ion dope using stoppers 517 located above the channels 515,516 as masks. In this example, the TFT drawn towards the right of the figure is a n-type channel TFT having impurity ions such as phosphorus (P) implanted in source 518 and drain 519, while the TFT on the left is a p-type channel TFT having impurity ions such as boron (B) implanted in source 521 and drain 520.
An interlayer insulating film 522 formed by sequentially depositing a SiO2 film, a SiN film, and a SiO2 film is provided on the entire surface over the gate insulating film 512, the active layer 513, and the stoppers 517. Contact holes formed in the interlayer insulating film 522 in regions corresponding to the sources 518,521 and the drains 519,520 are filled with metal such as Al, forming source electrodes 523,525 and a drain electrode 524. The drain electrode 524 connected to the drains 519,520 is provided in common for the n-type channel TFT and the p-type channel TFT. A planarizing insulating film 526 made of an organic resin is formed over the entire surface for planarization.
Above this, the magnesium-indium alloy cathode 167 of the organic EL display element 161 illustrated in FIG. 3 is formed over the entire surface.
The inverter 500 composed on an n-type channel TFT and a p-type channel TFT is configured as described above. The other inverter 400 has a similar structure.
In the manner described above, an organic EL display device comprising a horizontal drive circuit with inverters 400,500, a vertical drive circuit, and a display pixel can be created.
However, when the cathode 167 of the organic EL element 161 is provided on the entire surface over the peripheral drive circuit region and the display pixel region of the organic EL display device as described above, a back channel is created in each TFT because of the cathode 167. The existence of a back channel unduly influences the device, especially in the TFT of the peripheral drive circuit having a C-MOS structure. This is explained below.
FIG. 7 shows Vg-Id characteristics of n-type and p-type channel TFT. In the figure, the dotted lines indicate the initial characteristics, while the solid lines indicate characteristics after power is switched on.
As shown in the FIG. 7, there is at first no current leakage in either the n-type or p-type channel TFT when the gate voltage Vg is 0 V. However, when power is turned on, the potential applied to the cathode causes the characteristic of p-type channel TFT to shift to the right and the characteristic of n-type channel TFT to shift to the left. As a result, current leaks in both TFTs when Vg=0 V.
In a peripheral drive circuit, the TFT has a complementary structure composed with a p-type channel and an n-type channel. Accordingly, a change in the threshold voltage of the p-type channel TFT is caused when a high voltage is applied, while a change in the threshold voltage of n-type channel TFT is caused when the signal voltage is low. As a result, current flow, namely, a penetration current, is generated even when the gate voltage Vg=0. Generation of penetration current due to such changes disadvantageously causes an increase in power consumption.